The ever increasing demands for high capacity communications systems has resulted in a wide spread deployment of optical fiber networks across the world. A fundamental component used in such systems receives pulses of light and converts these into electrical signals. The pulses of light in such systems comprise a bit stream of information. This fundamental component employed in the fiber optic networks is commonly known as an optical receiver module. Within the optical receiver, a photodetector is typically employed to receive the light pulses and an amplifying circuit is employed for amplifying photocurrent generated within the photodetector.
Transimpedance amplifiers (TIAs) are typically used within optical receiver modules to amplify and transform weak photocurrents received from the photodetector, in the form of a photodiode or a PIN diode. The TIA amplifies and transforms the photocurrent into an output voltage that is further provided to other stages of the optical receiver module. Since TIAs are used to deal with both strong and weak photocurrents, noise in the resultant amplification and transformation to a voltage signal is typically a problem. Indeed, for those skilled in the art of the design of TIAs, it is well understood and appreciated that the noise introduced by the TIA, in many circumstances, limits the ability of the optical receiver module to faithfully reconstruct the intended stream of information. Furthermore, a relationship between the rate at which the receiver produces errors—often called the Bit Error Rate (BER), and the noise generated by the TIA can be shown. Thus, the optical receiver module needs to have low noise amplification performed on the weak photocurrents in order to facilitate optical transmission of information. This is especially true in circumstances where the distance that the optical signal must travel is long and results in weak optical pulses at the receiver. It is known to those skilled in the art that long transmission distances—the distance between a transmitter and a receiver—serves to attenuate the initial transmitted optical signal strength and places a greater burden upon the receiver module to avoid errors. Furthermore, it is also known that cost of an optical communication system is reduced if a signal is transmitted along a longer length of optical fiber or, in the alternative, if less optical power is transmitted. Thus, providing low noise amplification for the TIA is important in order to reduce the bit error rate (BER) of the received and amplified signal.
However, TIAs must not only provide low noise amplification of weak photocurrents, but must also operate when a much higher optical power is received by the photodetector and hence a much higher photocurrent is provided to an input port of the TIA. Thus, the TIA must exhibit wide dynamic range operation so that it does not suffer from input photocurrent overload, where the output voltage from the TIA is no longer linearly proportional to the input photocurrent. For example, if a TIA is used with a short transmission length of optical fiber, then the optical signal power levels received by the photodetector and amplified accurately can be much higher than when the TIA operates with much longer transmission lengths. Of course, with the higher photocurrent received from the photodetector, the TIA must also exhibit acceptable BER performance as with the lower photocurrents.
In practice, in order to achieve a wide dynamic range for TIA operation, some form of switching circuit is typically used, or in some cases, an AGC is utilized in order to vary the transimpedance gain of the TIA. In U.S. Pat. No. 6,218,905, entitled “Common-gate transimpedance amplifier with dynamically controlled input impedance,” an AGC is utilized in order to vary the gain of the TIA. In U.S. Pat. No. 6,297,701, entitled “Wide dynamic range transimpedance amplifier,” an AGC function is realized by a transistor switching network.
The most commonly used photodiode detector is the PIN diode, were the anode is usually connected to the input of the TIA while the cathode is connected to the positive voltage rail. Depending upon the type of PIN diode used, the wavelength of operation and possibly the data rate, the amount of reverse voltage required to allow the PIN diode to operate within its full dynamic range—from maximum sensitivity to overload—can vary between 5V to 0.8V. The reverse bias voltage requirement often dictates the circuit architecture of the front end transimpedance stage, which provides the input bias voltage to the PIN diode. As a result, performance compromises between sensitivity and overload typically occur.
If high optical sensitivity is required from the receiver, the TIA input bias voltage is required to be as low as possible (0.8V), which provides maximum reverse bias voltage, thus the PIN diode exhibits minimum detector capacitance. However, this design approach reduces the overload performance of the TIA. On the other hand, if a higher overload is required, a TIA requires a higher input bias voltage, which in turn reduces the PIN diode reverse bias voltage, increasing the detector capacitance and reducing the optical receiver's sensitivity. The issue is exacerbated if a further requirement is to provide a 3.3V single rail operation, which can often restrict the input bias voltage required to offer both high sensitivity and overload performance from the TIA.
A need therefore exists to provide a high reverse bias voltage required by a PIN diode operating at 1300-1550 nm wavelength and 10 Gbit/s from a 3.3V single supply rail and to providing sufficient reverse bias voltage to allow full dynamic range to be achieved without significant compromise in sensitivity and overload performance of the TIA.
It is therefore an object of the invention to provide a TIA that offers wide dynamic range operation without resulting in a significant compromise between sensitivity and overload performance. It is a further object of the invention to provide a current source for biasing of the TIA.